Listening to various adventitious or abnormal breathing sounds has proven to be an important diagnostic tool for detecting and monitoring certain types of lung diseases. Abnormal pulmonary sounds are generally detected by placing a stethoscope over selective areas of a patient's chest and listening for the sounds directly. The type of abnormal sound, its location, and its frequency of occurrence are used to make determinations of the type of disease and its severity.
The detected sounds are typically classified into normal lung sounds or adventitious (abnormal) sounds, usually divided into continuous or discontinuous sounds depending on their duration. Continuous sounds are further divided into wheezes, which are high-pitched, hissing sounds and rhonchi, which are low-pitched, snoring sounds. Discontinuous sounds are similarly divided into coarse crackles, which are short intermittent explosive sounds having a low pitch, or fine crackles, which are distinguished from coarse crackles in that they are less loud, shorter in duration, and higher in pitch. Other adventitious sounds include pleural friction rub and bronchial breathing.
Generally, it is a difficult task for an observer to detect accurately various lung sound abnormalities, since they are frequently of short duration, sometimes of relatively low amplitude, and generally mixed in with normal breathing sounds, which sometimes obscure the abnormal sounds. Furthermore, the task of classifying, quantifying and documenting lung sounds is difficult to accomplish with a stethoscope. Observers vary greatly in their abilities in this regard, making diagnosis less reliable.
An apparatus which forms visual waveforms representing the breathing sounds of a patient using a time-expanded scale has been disclosed by the present inventor in U.S. Pat. No. 3,990,435. This apparatus permits a trained observer to visually delineate the abnormal sounds from normal sounds. This has greatly improved diagnostic accuracy and helped clarify the previously confused lung sound nomenclature. Diagnosis, however, depends on the observer to properly sort and distinguish the different abnormal lung sounds from the normal lung sounds detected at various locations. The manual sorting of lung sounds visually or acoustically is a tedious task whose accuracy depends on the experience and alertness of the observer.
Another method of adventitious sound detection employs one or more bandpass filters to selectively detect sounds in preselected frequency ranges. Abnormal sound identification is based on the percentage of the total sample made up of sounds in the selected ranges. This technique is very unreliable; many of the adventitious sounds have components in the same or overlapping frequency ranges. As a result, the technique often cannot distinguish the different types of sounds, especially when more than one type is present.